The present invention, in some embodiments thereof, relates to a controllable optical sensor and, more particularly, but not exclusively, to an apodized optical device which is controllable, in one example to provide defined patterns and in another example to provide an optical 3D device for determining and tracking the positions in three dimensional space of objects in view and/or determining their 3D structure, and in a third example for an optical beam expander.
Current methods for optical 3D sensing and motion tracking consist of mainly two technologies, time-of-flight (TOF) and structured illumination.
TOF uses a similar concept as in range finding technology, where a light pulse is illuminated towards an object surface (generally from an IR emitter) and photons reflected from the object surface are collected via a sensor, generally a unique CMOS sensor with a very high rate of acquired frames per second designed exclusively for these purposes. By measuring the time from the triggered light pulse until the reflected photons are received by the sensor, it is possible to calculate the distance and effectively the location in space of the object surface. By emitting and measuring multiple light pulses in many directions it is possible to extract a 3D mapping of the targeted space.
One limitation of this method is the inability to obtain depth sensing on objects at close proximity to the depth sensor. General operating ranges are known to be above one meter from the depth sensor. Thus gesture recognition for controlling the computer cannot be provided from a short distance, which is the distance a person normally uses a computer. Various techniques can be applied to overcome this limitation but they require additional methods and hardware components thus increasing the complexity of design and price.
An additional method for depth sensing comprises of illuminating the surface of an object with structured light, also known as a light pattern. The light is generally illuminated by an IR source emitting light through a diffractive optical element (DOE). A sensor located at some distance from the light source is used for collecting photons reflected from the object surface and by means of triangulation calculations, the distance from the surface is determined and a 3D mapping of the illuminated space is generated.
Gaming applications and the like, require a high 3D resolution of motion detection and tracking. In order to achieve the required resolution the projected structured light pattern is very dense. The density limits the method from operating in a full dynamic range of proximities. In order to effectively operate on distant objects the projected pattern is designed to appear dense. But when operating at close proximities, generally under 0.8 m, the projected features overlap and do not provide the pattern that is required for generating the necessary calculations for depth detection and motion tracking.
In order for structured light depth sensing of objects at a closer proximity range, it is possible to design a DOE such that the projected pattern is less dense at closer proximities. This design provides the required characteristics for discriminating between features of the projected pattern. For example, a random spots design that contains fewer spots allows for the separation between spots closer to the sensor. In these types of designs however, the resolution deteriorates throughout the distance since the spots diverge from each other, resulting in poor resolution at these larger distances.
Another option for discriminating between features is by using a lens with a relatively close focal point. However, in this type of setup the features at distant proximities are defocused, blurred and overlap, thus resulting in the same problems as with the first design but for distant proximity ranges.
Both of the mentioned methods are thus restricted to specific operating distances and no current method offers a solution for a single depth sensor to effectively operate on a full dynamic range of distances.
FIG. 1 illustrates a particular light pattern and an attempt to view it over a wide distance range. As can be seen in the images, the pattern features that are clear and discrete at a distance of 900 mm cannot be resolved at 300 mm and all that appears at 150 mm is a blur. Depth sensing cannot thus be performed at the smaller range using this system.
Other methods use various types of projectors to project images onto the object inspected in order to extract the 3D information. However, these types of imaging methods require an imaging lens to keep the image relatively in focus enabling the detection of the depth map around the focal plane of the projector. The need for focus again limits the dynamic range of operation of the device.